Horizontal and vertical wavenumbers (k_x, k_z) immediately below the Ozmidov wavenumber are spectrally distinct from both isotropic turbulence (k_x, k_z > 1 cpm) and internal waves as described by the Garrett-and-Munk (GM) model spectrum (k_z < 0.1 cpm). Towed CTD chain, augmented with concurrent EM-APEX profiling float microstructure measurements and shipboard ADCP surveys, are used to characterize 2D wavenumber (k_x, k_z) spectra of isopycnal slope, vertical strain and isopycnal salinity-gradient on horizontal wavelengths of 50 m – 250 km and vertical wavelengths of 2 – 48 m. For k_z < 0.1 cpm, 2D spectra of isopycnal slope and vertical strain resemble GM. Integrated over the other wavenumber, the isopycnal slope 1D k_x spectrum exhibits a roughly + 1/3 slope for k_x > 3 x 10^-3 cpm, and the vertical strain 1D k_z spectrum a –1 slope for k_z > 0.1 cpm, consistent with previous 1D measurements, numerical simulations and anisotropic stratified turbulence theory. Isopycnal salinity-gradient 1D k_x spectra have a + 1 slope for k_x > 2 x 10^-3 cpm, consistent with nonlocal stirring. Turbulent diapycnal diffusivities inferred in the (i) internal-wave subrange using a vertical strain-based finescale parameterization are consistent with those inferred from finescale horizonal wavenumber spectra of (ii) isopycnal slope and (iii) isopycnal salinity-gradients using Batchelor model spectra. This suggests that horizontal submesoscale and vertical finescale subranges participate in bridging the forward cascade between weakly nonlinear internal waves and isotropic turbulence, as hypothesized by anisotropic turbulence theory.

Microstructure and CTD/LADCP measurements from the Western Mediterranean basin east of 5°E revealed two types of dynamical regions (Ferron et al., 2017), contrasted in terms of current magnitude, vertical shear, stratification and turbulent kinetic energy dissipation rate: energetic regions (Corsica Channel, Egadi Valley and Sicily Channel) and quiescent regions (Ligurian Sea, around Sardinia, and Tyrrhenian Sea). On average, the current speed and the buoyancy frequency in the energetic regions were twice as large as in the quiescent regions, and the vertical shear was five times as large. Turbulence properties inferred from the microstructure measurements were also contrasted, dissipation rates in the energetic regions being two orders of magnitude larger than in the quiescent regions. The present study investigates the variability of the dissipation flux coefficient, a measure of the mixing efficiency, in a rich assortment of dynamical regimes. This dataset covers the full range of turbulence intensities observed in previous studies based on field measurements, direct numerical simulations, and laboratory experiments alike. The dependency of the dissipation flux coefficient as a function of turbulence intensity for the quiescent and energetic regions frames the previously observed lower and upper bounds, respectively. A contrasting behaviour was revealed between the two types of regions. In the quiescent regions, the dissipation flux coefficient linearly decreases on average by one order of magnitude with turbulence intensity increasing by four orders of magnitude. On the other hand, in the energetic regions the dissipation flux coefficient exhibits a nearly constant value over 4 decades of turbulence intensity, before decreasing for very strong turbulence intensities. In contrast with other studies, this dataset shows no relationship between the Richardson number and the dissipation flux coefficient. This may be due to inadequate vertical sampling resolution of the currents, or to the high diversity of sampled turbulent regimes, contrary to previous studies focused on a single type of dynamical region or framework (such as the thermocline or shear instabilities).

The dissipation flux coefficient, a measure of the mixing efficiency of a turbulent flow, was computed from microstructure measurements collected with a vertical microstructure profiler in the Sicily Channel. This hotspot for turbulence is characterised by strong shear in the transitional waters between the south-eastward surface flow and the north-westward deep flow. Observations from the two deep passages in the channel showed a contrast in turbulent kinetic energy dissipation rates, with higher dissipation rates at the location with the strongest deep currents. This study investigated the dissipation flux coefficient variability in the context of mechanically driven turbulence with a large range of turbulence intensities. The dissipation flux coefficient was shown to decrease on average with increasing turbulence intensity Re_b, with median values of 0.74 for low Re_b (< 8.5), 0.48 for moderate Re_b (8.5 ≤ Re_b < 400) and 0.30 for high Re_b (≥ 400). The dissipation flux coefficient inferred from the measurements was systematically higher on average than the parameterisation as a function of turbulence intensity suggested by Bouffard and Boegman (Dyn Atmos Oceans 61:14–34, 2013). A plateau at moderate turbulence intensities was observed, followed by a decrease in the dissipation flux coefficient with increasing turbulence intensity as predicted by the parameterisation, but at higher turbulence intensity. The dissipation flux coefficient showed a strong variability with the water column stability regime for the different water masses. In particular, high dissipation flux coefficient (median 0.40) was found at Re_b between 400 and 104 for the transitional waters at the northeastern passage, where dissipation rates were high, stratification and shear were strong but the Richardson number Ri was sub-critical. Vertical diapycnal diffusive fluxes were computed, and upward salinity sustained density fluxes of the order of 9 x 10^-6 and 4 x 10^-6 kg m^-2 s^-1 were found to be characteristic of the transitional (28 < σ < 29 kg m^-3) and intermediate (σ > 29 kg m^-3) waters, respectively. Turbulent mixing led to a lightening of the transitional and intermediate waters, which was consistent with previous estimates (Sparnocchia et al. J Mar Syst 20:301–317, 1999), but an order of magnitude lower when inferred from the (Bouffard and Boegman Dyn Atmos Oceans 61:14–34, 2013) parameterisation.